The present invention relates to a novel copolymer and the preparation thereof, and more particularly to a copolymer suitable for use in soft contact lenses.
The present invention also relates to soft contact lenses having low water content which can be worn continuously for a long period of time without pain, and more particularly to low water content soft contact lenses made of a novel copolymer having an excellent oxygen permeability which is increased by introducing a siloxane bond at branching chain ends of a polymer chain.
Typical examples of known soft contact lenses developed with the object for continuous wear for a long term are silicone rubber contact lenses prepared from polydimethyl siloxane as a main component and high water content contact lenses prepared from poly-N-vinyl pyrrolidone as a main component.
Since the silicone rubber contact lenses are very water-repellent and greatly different from cornea in thermal properties such as thermal conductivity and thermal diffusivity, they give a foreign body sensation, particularly a burning sensation despite having a large oxygen permeability. Therefore, there is required a strong patience to accommodate to the silicone rubber lenses. Further, the silicone rubber is so soft and elastic that they are subjected to precise mechanical treatments such as cutting, grinding and polishing with difficulty. Also, many attempts for making the surface of silicone rubber lenses hydrophilic have been undertaken, but no satisfactory contact lense has been developed.
Since the high water content contact lenses contain about 60% to about 80% by weight of water, they have the disadvantages that (1) they are weaker in quality of material than low water content contact lenses, (2) they are easily contaminated with inorganic and organic compounds in tears which penetrate and accumulate into the lenses during the use and (3) they are bad in maintenance of lens contour due to the evaporation of water during the use and, therefore, the refractive power thereof easily changes.
Also, there is proposed soft contact lenses prepared from polyethyleneglycol monomethacrylate (i.e. poly-2-hydroxyethyl methacrylate) as a main component. However, the water content of these contact lenses is usually at most 40% by weight and the oxygen permeability is insufficient. Therefore, these contact lenses have the defect that they cannot be worn continuously for a long time.
As stated above, conventional soft contact lenses are not necessarily suitable for continuous wear for a long term.